The present invention relates to fibers for reinforcing matrix materials such as mortar, concrete, shotcrete, rubber, plastic, bituminous concrete, gypsum compositions, or asphalt and more particularly to fibers having a three-dimensional twist for enhancing dispersibility of fibers within mortar and concrete.
The present invention particularly focuses on the problem of dispersing fibers within castable compositions such as fresh cementitious mixes. The problems associated with adding fibers to concrete and avoiding fiber clumping or balling is well documented in xe2x80x9cGuide for Specifying, Proportioning, Mixing, Placing, and Finishing Steel Fiber Reinforced Concretexe2x80x9d (Document number ACI 544.3R-93) as reported by American Concrete Institute Committee 544.
Over the years numerous innovative methods of packaging and bundling fibers, surface treatments and mechanical means of adding fibers to a concrete mixture have been devised in order to try and overcome the problem of fiber balling or clumping. U.S. Pat. No. 4,121,943 (Akazawa et al.) describes a machine designed to separate fibers into separate units prior to adding them to a concrete mixture. U.S. Pat. No. 3,716,386 (Kempster) describes a process whereby the fibers are coated with a friction reducing substance prior to their introduction to a concrete mixture. U.S. Pat. Nos. 4,224,377 and 4,314,853 (Moens) describe a method whereby a plurality of wire elements are united by a binder which loses its binding ability during the mixing process. U.S. Pat. No. 5,807,458 (Sanders et. al.) describes a method for reinforcing castable compositions through the use of reinforcing elements maintained in a close-packed alignment in a dispersible containment means.
A factor common to the last two described methods for achieving high addition rates of high aspect ratio fibers into concrete is to introduce the fibers in a organized array that on mixing slowly release the fibers in an aligned array. Fibers released into cementitious compositions in this manner experience fewer fiber-fiber interactions and subsequently show less tendency to clumping or balling as compared to the same fibers that are added to cementitious compositions in a totally random orientation. This fiber clumping or balling means that the individual fiber strands do not disperse uniformly throughout the concrete mix, and therefore they may fall short of imparting the desired structural reinforcement to the resultant hardened concrete matrix or unit as a whole. One significant drawback of the last two described methods is that the release of the fibers in the mixing unit is dependent on the rate at which the dispersible containment or the binder will dissolve. Therefore, mixing operations involving short mixing cycles may not allow enough time for the entire release of the fibers from the dispersible containment or the total dissolution of the binder uniting the fibers. The fibers in the present invention do not rely on any binding agent or dispersible containment for proper release and dispersion and can therefore be used in operations where short mixing cycles are involved.
The present invention covers fibers that can be rapidly added to a cementitious composition in a completely random orientation with no fiber clumping or balling occurring.
The present invention provides fibers for reinforcing matrix materials such as hydraulic cementitious materials (e.g., mortar, concrete). Exemplary fibers of the invention have an average length of 5-100 mm, an average width of 0.25-8.0 mm, and first and second opposed ends each having width and thickness dimensions with width dimensions exceeding thickness dimensions, said widths of said first and said second opposed ends being twisted and thereby having different orientations. Preferably, the widths are oriented in directions that are non-coplanar with each other, and more preferably between 15xc2x0-720xc2x0 out of phase with each other (the upper number representing two complete twists) and more preferably between 15xc2x0-360xc2x0 out of phase with each other. The fibers can be made of one or more synthetic polymers (e.g., polypropylene, polyethylene, etc.), steel, or other materials.
In other exemplary embodiments, the fibers have intermediate body portions defined between the first and second opposed fiber ends which have a three-dimensional curve or twist. For example, if the fiber body when stretched into a straight line is deemed to occupy the xe2x80x9czxe2x80x9d axis, then the fibers can be deemed to have a curve in the xe2x80x9cxxe2x80x9d direction (defined along the width dimension of the fibers), as well as a curve in the xe2x80x9cyxe2x80x9d direction (defined along the thickness dimension of the fibers).
The curvature of the fibers can be mathematically described using the following equation:       (                            x                                      y                                      z                      )    =            (                                                  a              ·                              cos                ⁡                                  (                                                            ω                      1                                        ·                    τ                                    )                                                                                                        b              ·                              sin                ⁡                                  (                                                            ω                      2                                        ·                    τ                                    )                                                                                                        v              ·              τ                                          )        ⁢          xe2x80x83        ⁢                                                      xe2x80x83                        ⁢                                          ω                1                            ,                              ω                2                            ,              v              ,                              τ                ∈                ℛ                                                                                                                    0.25                ⁢                mm                            ≤              a                        ,                          b              ≤                              25                ⁢                mm                                                        
To obtain a full twist for a given length l in z direction the following equation has to be fulfilled:   v  =            l      ·              (                              ω            1                    +                      ω            2                          )                            4        ·        P            ⁢              xe2x80x83            ⁢      i      
A preferred process for making the aforementioned exemplary fibers of the invention comprises twisting together 2-5000 and more preferably 6-24 fiber strands (each strand of which can be a monofilament, multifilament, or which in turn can comprise further strands), and then cutting the twisted fiber bundle into separate fiber pieces which will have the twisted structure, as described above. The memory of the twist shape is generally maintained in the fiber material after cutting into separate fiber pieces. The memory of the twist shape in the fiber material can be enhanced by introducing the twisted fiber bundle (before cutting) against and around one or more pulleys to impart tension on the twisted fiber bundle material, or such as by introducing the twisted fiber bundle between rollers to flatten or crush them or otherwise to impart the twisted shape into the memory of the fibers. Heating of the twisted fiber bundle before cutting can also impart the twisted shape into the memory of the fibers.
Preferably, the plurality of fibers made in accordance with the invention will have curvatures or arches retained in the material memory (slightly bent portions between opposing ends of the fiber) that vary from fiber to fiber, and this can be achieved depending upon the nature of the material (polymer, steel, other) and number of twists per fiber length, such as 1-96 twists and more preferably about 6-8 twists per linear foot of fiber. The cutting of fibers when in a twisted-together state surprisingly provides a plurality of fibers that have different curvatures as well as opposing cut ends that can veer off in different directions.
The unique twisted structure of the resultant fibers enhances the dispersibility of the fibers in a matrix composition such as concrete. In the present invention, the ability to impart a curvature as a result of the twisting will generate a plurality of individual fibers having different curvatures (because at any given point on the twisted fiber bundle the individual fibers will have different curvatures) as well as different bias properties. The different bias properties arise because the curves or bends arise at different portions of the fiber length, and the bias properties are such that the fibers are naturally biased away from each other after the cutting process. The inventors believe that the variable bias created by the varying curvatures in the twisted fibers helps to separate the individual fibers after they are introduced into the matrix composition.
Twisting-fibers together provides numerous other advantages and benefits. One such advantage is the convenience of processing a high number of fiber strands at once through a cutter at a high rate of speed. The twisting of the strands also provides convenience in handling.
In addition to fibers and processes for making them, the invention also provides methods for modifying cementitious compositions, such as by introducing the above-described fibers into wet concrete or mortar. The invention is also directed to hydratable cementitious compositions having the above-described fibers.
Further advantages and features of the exemplary fibers, methods, and compositions of the invention may become more apparent in discussion hereinafter.